1. Technical Field
The present invention relates to electrodes for electric double layer capacitors which are suitable for use for electric double layer capacitors having large capacity and high power.
2. Background Art
An electric double layer capacitor has characteristics such as long service life, high cycle characteristics, and characteristics of charge and discharge with heavy current since there are no chemical reactions during charge and discharge of the capacitor as there are in a conventional secondary battery. Therefore, this capacitor is attracting much attention as a new type of storage battery or as a driving power supply automobiles and devices. In particular, an electric double layer capacitor having large capacity and high power is being developed.
As a process for producing such an electric double layer capacitor, a method in which conductive adhesive is coated on a collector sheet, such as aluminum foil, and an electrode forming sheet is joined through this conductive adhesive, is known (see Japanese Unexamined Patent Application Publication No. 11-162787). In this method, activated carbon, conductive carbon, binder, and solvent are mixed and kneaded first, and the electrode forming sheet is obtained by rolling and drying. A conductive filler such as carbon black or graphite and a binder comprising a resin component such as polyvinyl alcohol (PVA) are mixed to prepare a conductive adhesive, and then the conductive adhesive is coated on the collector sheet and is joined with the electrode forming sheet and is dried.
It is known that carbon black, which has particles of small diameter, and graphite, which has particles of large diameter, are used together as this conductive filler in order to maintain adhesive strength by increasing contact surface area with small particles, and in order to maintain macro conductive paths with large particles (see Japanese Unexamined Patent Application Publication No. 7-201663.
Furthermore, since water-based solvent is used in the conductive adhesive using PVA in the above-mentioned process for production of the electrode for electric double layer capacitor, preparation is easy, it is not necessary to use harmful organic solvents, reducing environmental measures, the cost is low, and durability is superior.
However, there is a problem in that the initial resistance of the capacitor cannot be restrained only by using carbon black and graphite together, and there is also a problem in that adhesive strength is not improved. There are even cases in which efficiency is deteriorated. It should be noted that there is no description of the kind of carbon black and graphite which may be used in the above patent application publication. Furthermore, in the case in which it is used as a driving power source of a car, the temperature of the electrolyte is increased since the capacitor is used under severe conditions, and there is a problem of interface separation caused by deterioration of adhesive strength.
Furthermore, there is a problem in that a water based adhesive such as PVA may be easily affected by, for example, surface properties of adhering material or temperature, and adhesive strength may easily become unstable. If adhesive strength becomes unstable, the collector sheet and a layer comprising conductive adhesive (hereinafter referred to as a conductive adhesive layer) may be easily separated along the interface, and the interface resistance is increased while charging and discharging. In particular, in a case in which the capacitor is used as a driving power resource of a car, the temperature of the electrolyte is increased under severe conditions, and interface separation may easily occur.